Method of manufacturing a luminescent silicate activated by bivalent europium

ABSTRACT

A luminescent silicate activated by bivalent europium defined by the formula Ba1-x-pSrxEupZrSi3O9 which O&lt;/= x &lt;/= 0.2 and 0.002&lt;/= p &lt;/= 0.10 is prepared by starting from a mixture of barium strontium disilicate, at least 1.5 mol of ZrO2 and at least 1 mol of SiO2 per mol of disilicate. The mixture, which also comprises europium, is heated at a temperature of between 1,200 DEG   and 1,400 DEG C is a reducing atmosphere.

I Umted States Patent 1 [111 3,718,600 Tak et al. [4 1 Feb. 27, 1973METHOD OF MANUFACTURING A, References Ci e LUMINESCENT SILICATEACTIVATED UNITED STATES PATENTS BY BIVALENT EUROPIUM 2,l7l,l45 8/1939Leverenz ..252/301.4 F [75] Inventors: Marinus Gerardus Antoine Tak;2,297,108 9/1942 McKeag et al. ..252/301.4 F

Otto Jan Van Klinken, both of Emmasinge], Eindhoven, Netherlands FOREIGNPATENTS OR APPLICATIONS 73 Assignee: s Philips Corporation, New1,915,360 10/1969 Germany ..252/30L4 F York, N.Y. Primary ExaminerRobertD. Edmonds Filed! J 1971 Attorney-Frank R. Trifari [211 App]. No.:155,840

[57] ABSTRACT [30] Foreign Application priority Data A luminescentsilicate activated by bivalent europium defined by the formula Ba,,Si',Eu,,ZrSi O which Os July 1, 1970 Netherlands ..7009688 x s 02 and0002 s p s 10 is prepared by Start ing from a mixture of bariumstrontium disilicate, at [52] US. Cl ..252/30L4 F least 1.5 mol of 21.02and at least 1 mol of sio2 per Int. Cl. ..C09k l/54, C09k 1/62 mo] ofdisilicate' The mixture which also comprises Field of Search "252/3014 Feuropium, is heated at a temperature of between l,200 and 1,400C is areducing atmosphere.

7 Claims, 1 Drawing Figure PATENTEII 3,718,600

INVENTORS MARINUS G.A. TAK

OTTO J. VAN KLINKEN METHOD OF MANUFACTURING A LUMINESCENT SILICATEACTIVATED BY BIVALENT EUROPIUM This invention relates to a method ofmanufacturing a luminescent silicate activated by bivalent europiumwhich is defined by the formula Ba Sr,,Eu,,ZrSi O, in which s x s 0.2and 0.002 s p s 0.10. Furthermore the invention relates to a luminescentsilicate manufactured by such a method.

The luminescent barium zirconium silicates in which a portion of bariummay be replaced by strontium and which may be defined by theabove-mentioned formula have been described in the Netherlands Pat.application No. 6,916,882. These silicates can be satisfactorily excitedboth by short-wave and by long-wave ultraviolet radiation and even byblue visible radiation. Also when excited by electrons they have asatisfactory light output. The emitted radiation of these silicatesconsists of a narrow band having a maximum at approximately 475 nm. As aresult of the high light output and the favorable location of theemission in the spectrum these silicates are particularly suitable foruse in lowpressure and high-pressure mercury vapor discharge lamps.

The above-noted Netherlands Patent application describes a method ofmanufacturing these silicates while starting from a mixture of bariumcarbonate (and optionally strontium carbonate), europium oxide,zirconium oxide and silicon oxide. A portion of barium and/or strontiummay furthermore be added as a fluoride. These starting materials areused in quantities corresponding to the quantities required inaccordance with the stoichiometry except for a small excess of sinteredproduce is obtained so that long periods of 5 grinding operations arerequired for practical uses. This dense sintering also occurs when thefiring mixture is first prefired at comparatively low temperatures andis subsequently after-fired at a comparatively high temperature. It hasbeen found that in connection with the occurrence of the dense sinteringof the final product in the method only firing temperatures can beemployed at which the full conversion of the starting materials into thedesired silicate phase is impossible. A full reaction is possible atvery high reaction temperature (for example, to 1,450C). The bariumzirconium silicate formed is then, however, very densely sintered andhas often even melted so that it is not suitable at all for practicaluses.

The object of the present invention is to provide an improved method ofmanufacturing the luminescent silicates mentioned in the preamble inwhich the drawbacks of the method already described are largelyobviated.

According to the invention a method of manufacturing a luminescentsilicate activated by bivalent europium defined by the formula BASr,,.Eu,,ZrSi,O in

whichO g x S 0.2 and 0.002 S p 5 0.10is characterized in that bariumstrontium disilicate is mixed with at least 1.5 mol of ZrO, per mol ofdisilicate and at least 1 mol of SiO, per mol of disilicate, europiumbeing added in the desired quantity to the mixture, and that the mixtureis subsequently heated at a temperature of between l,200 and 1,400C in areducing atmosphere.

In a method according to the invention barium strontium disilicate isused as a starting material. In this respect the term barium strontiumdisilicate is to be understood to mean barium disilicate in which bariummay be partly replaced by strontium, namely for as large a part as isdesired in the final product. Furthermore, this term comprises a mixtureof barium disilicate and strontium disilicate in the desired molarratio. Finally it is possible that barium strontium disilicate comprisesthe quantity of europium or a part thereof desired in the final product.Further starting materials in a method according to the invention areZr0 and SiO, or compounds which produce these oxides upon heating.Europium may be used as Eu o but it is alternatively possible to use acompound which produces europium oxide upon heating or, as alreadydescribed above, to start from a europium-containing barium strontiumdisilicate.

SiO is to be present in a quantity which is at least equal to thestoichiometrically required quantity (1 mol per mol of disilicate). Onthe other hand ZrO is to be used in an excess amount in a methodaccording to the invention, namely at least 1.5 mol per mol ofdisilicate (according to stoichiometry, only 1 mol per mol of disilicateis required).

In a method according to the invention it is found to be possible to usecomparatively high firing temperatures (up to 1,400C) at temperatures afinal product is formed which is only slightly sintered. By startingfrom barium strontium disilicate a full reaction of the firing mixturein the desired luminescent silicate phase may be achieved with no densesintering occurring. The absence of other luminescent silicate phases isfound from the spectral distribution of the emission of the reactionproduct upon excitation by ultraviolet radiation. It has suprisinglybeen found that when 'using an excess of ZrO in the firing mixtureluminescent silicates are obtained whose light outputs are considerablyhigher than those of the silicates obtained from a firing mixturecontaining the stoiciometric quantity of ZrO,,. The excess of ZrOremains in the final product and causes no disturbing effects.

In addition to the advantages of the reduced amount of sintering and ofthe high light output of the silicate obtained, a method according tothe invention has the important advantage that upon excitation byultraviolet radiation the silicate obtained is found to have a spectraldistribution of the emitted radiation which is nar- 3.0 mol of ZrO permol of disilicate. These quantities of ZrO, are therefore preferred.

In a further preferred embodiment of a method according to the inventionSiO is also used in excess, namely 1.5 to 2.5 mol of SiO, per mol ofdisilicate is added. A reaction product is then obtained which is verymuch desired for the processing of the reaction product. The use of anexcess of SiO substantially does not exert any influence on theluminescence properties of the final product.

The time during which the heat treatment is to take place for a methodaccording to the invention may be chosen to be within very wide limits.This heating time is dependent on the chosen firing temperature, thereactivity of the starting mixture and the quantity of the startingmixture. In general firing times of between 2 and 8 hours are preferredat firing temperatures of between l,300 and 1,400C. It is oftenadvantageous to carry out the heat treatment in steps, for example, 2 ormore times for 2 to 4 hours at a temperature of between l,300 and1,400C. After each heat treatment the product obtained is ground andsieved if necessary.

The barium strontium disilicate to be used in a method according to theinvention may be previously prepared in known manner by heating astarting mixture of BaO, SrO or SiO, or of compounds which produce theseoxides upon heating. In a preferred embodiment of a method according tothe invention the disilicate is previously prepared by heating such amixture which contains at least 3 mol of SiO; per mol of barium oxideand strontium oxide, so that the disilicate formed is already mixed withthe quantity of Si required for obtaining the desired luminescentsilicate. In this manner an even slighter sintering of the final productis obtained so that the luminescent silicate can be processed evenbetter.

As already noted above, the barium strontium disilicate to be used as astarting material in amethod according to the invention may contain therequired quantity of europium. Europium is then built into thedisilicate lattice and there it replaces the barium or strontium ions.Such a starting material is, often preferred because, as compared with amethod in which europium is added as Eu O a higher light output of thefinal product is obtained. Depending on the firing atmosphere during themanufacture of europiumcontaining disilicate, europium is present in abivalent or trivalent state. If the disilicate contains trivalenteuropium it is reduced to bivalent europium during the formation of thedesired luminescent silicate by means of heating in a reducingatmosphere.

In order that the invention may be readily carried into effect, it willnow be described in detail with reference to several Examples, a Tableand a drawing.

In the drawing a graph shows the spectral energy distribution of abarium zirconium silicate according to the invention.

TABLE Firing mixture in moles Exarn- Light ple outalkaline earthdisilicate ZrO,

SiO, put in 96 a (2) 28.5 b (0.98Ba0) (2.lSi0,) l I 16.0

(4) (0.95Ba0) (3.lSiO,) 0.05Eu0) --w- N N EXAMPLE I For the manufactureof a barium zirconium silicate activated by bivalent europium accordingto the invention, an alkaline earth disilicate was used as a startingmaterial which comprises europium in a trivalent form and which isdefined by the formula (0.98Ba0) (2.1 SiO (0.01 Eu o manufactured byheating a mixture of barium carbonate, silicon dioxide an europium oxide(Eu o for 3 hours at 1,000C in air.

The disilicate thus obtained was mixed with l%mol ZrO and 1 mol SiO permol of disilicate and subsequently heated in a covered crucible for 4hours at a temperature of 1,350C. To obtain a reducing firing atmospherethe heat treatment was performed in the presence of a quantity of carbonor graphite in the vicinity of the crucible. The oxygen from theambiance was bound by the carbon so that a reducing atmospherecomprising carbon monoxide was formed. After the heat treatment, thecrucible with contents was allowed to cool off, while nitrogen waspassed into the cooling space (approximately 101 or N per minute). Thefiring product was pulverized, ground and sieved, if necessary,thereafter it was again subjected to a heat treatment under the samecircumstances as those described above, namely for 4 hours at atemperature of 1,350C in a reducing atmosphere.

The final product obtained satisfied the formula Ba Eu ZrSi O andfurthermore it contained the used excess of ZrO Upon excitation byultraviolet radiation having a wavelength of 254 nm the light output ofthe barium zirconium silicate thus prepared was 49.5 percent of thelight output of a barium strontium disilicate activated by bivalenteuropium and emitting in the same part of the spectrum.

EXAMPLES 2 TO 8 In the same manner as in Example 1, luminescentsilicates defined by the formula Ba Eu ZrSi O were manufactured inwhich, however, different quantities of ZrO, and SiO were used. Theabove-mentioned Table shows for each of the Examples 2 to 8 thequantities of ZrO, and SiO used per mol of alkaline earth disilicate.The last column of the Table shows for each Example the light outputupon excitation by ultraviolet radiation having a wavelength of 254 nm.The light output is given in percent relative to the luminescentdisilicate mentioned in Example 1, which is used as a standard.

For the purpose of comparison the Table includes the Examples b and c(not according to the invention). The manufacture of the luminescentsilicates according to Examples b and c is effected in a manner which iscompletely analogous to the manner as described in Example 1. However,in Example b the stoichiometrically required quantities of ZrO and SiO,have been used and in Example c only a slight excess of ZrO has beenused. It is quite obvious that with a method ac cording to the inventionluminescent silicates are obtained which have a considerable higherlight output than the silicates manufactured in accordance with Exampleb or c.

Likewise for the purpose of comparison a barium zirconium silicate hasbeen shown as Example a which is manufactured in accordance with themethod described in the Netherlands Pat. application No. 6,916,882 whichmethod starts from a mixture of 0.93 mol BaCO 0.05 mol BaF 1.0 mol Zr0,,3.3 mol SiO and 0.01 mol Eu O This mixture was heated for 4 hours at atemperature of 1,250C in a reducing atmosphere consisting of nitrogenand 5 percent by volume of hydrogen. After cooling, grinding and sievingthe reaction product was again heated in a reducing atmosphere for 2hours at 1,350C. The final product is relatively densely sintered andhas a light output of only 28.5 percent.

EXAMPLE 9 Entirely analogous to Example 8, a luminescent silicatedefined by the formula Ba Sr Eu ZrSi O was manufactured. The alkalineearth disilicate to be used as a starting material satisfies the formula(0.78

EXAMPLES 10 TO 12 The manufacture was effected in the same way asdescribed in Example 1, on the understanding that the alkaline earthdisilicate to be used as a starting material was manufactured by heatinga mixture of barium carbonate,silicon dioxide and europium oxide (Eu ofor 3 hours at a temperature of l,000C in a reducing atmosphere. Thedisilicate comprises europium in a bivalent form and satisfies theformula (0.98 BaO) (2.1 Si0 (0.02 EuO) Zr0 quantities of 2, 2% and 3mol, respectively, per mol of disilicate were used.

EXAMPLES 13 AND 14.

In the same manner as described with reference to Example 10, aluminescent barium zirconium silicate according to the invention wasmanufactured in which,

however, also 0.0025 mol Eu O (Example 13) and 0.005 mol Eu,o, (Example14) were added to the firing mixture. The luminescent silicates obtainedsatisfy the formulas Ba Eu ZrSi O and Ba Eu ZrSi 0 respectively.

EXAMPLE 15 By heating a mixture of barium carbonate, europium oxide andan excess of silicon dioxide at 1,200C in a reducing atmosphere analkaline earth disilicate was obtained which was already mixed with thequantity of silicon dioxide required for the formation of bariumzirconium silicate according to the invention. The resultant silicate,which may be defined by the formula (0.95 BaO) (3,1SiO (0.05Eu0), wasmixed with 2 mol zirconium oxide per mol of disilicate and wassubsequently heated twice in a furnace for 4 hours at a temperature of1,350C in a reducing atmosphere. The barium zirconium silicate obtainedsatisfies the formula Ba ,,,,Eu ZrSi O and has a light output of 60.5percent upon excitation by ultraviolet radiation.

The Table shows a survey of the composition of the firing mixture and ofthe light output for each Example measured upon excitation byultraviolet radiation (254 nm).

The Figure shows in a graph the spectral energy distribution of theemitted radiation of barium zirconium silicate manufactured inaccordance with Example 8 upon excitation by ultraviolet radiationhaving a wavelength of 254 nm (curve 8). The wavelength A in nm isplotted on the horizontal axis and the energy E of the emitted radiationis plotted in arbitrary units on the vertical axis. For the purpose ofcomparison the broken-line curve a is included which shows the spectralenergy distribution of barium zirconium silicate manufactured in knownmanner according to Example a. It is clearly shown that barium zirconiumsilicate manufactured according to the invention has a higher lightoutput and a considerably higher peak height than the silicatemanufactured in the known manner. Furthermore, it is found that the halfvalue width of the silicate manufactured in accordance with theinvention is smaller, (approximately 50 nm) than that of the silicatemanufactured in known manner (approximately 60 nm What is claimed is:

l. A method of manufacturing a luminescent silicate activated bybivalent europium defined by the 'formula Ba .,.,,Sr Ew,,ZNSi3O9 inwhich 0 x S 0.2 and 0.002 s p s 0.10, comprising mixing barium strontiumdisilicate with at least 1.5 mol of ZrO, per mol of disilicate and atleast 1 mol of SiO, per mol of disilicate, adding europium in thedesired quantity to the mixture and heating the mixture at a temperatureof between 1,200 and 1,400C in a reducing atmosphere.

2. A method of claim 1, wherein the mixture comprises between 1.75 and3.0 mol of ZrO per mol of disilicate.

3. A method of claim 2, wherein the mixture comprises between 1.5 and2.5 mol of SiO per mol of disilicate.

4. A method of claim 3 wherein the mixture is heated for 2 to 8 hours ata temperature of between 1,300 and 1,400C.

6. A method of claim 3 wherein the barium strontium disilicate to beused comprises the desired quantity of europium.

7. A luminescent silicate activated by bivalent europium and defined bythe formula l3a ,,Sr,Eu,,ZrSi O in which 0 S x s 0.2 and 0.002 s p s0.10, and manufactured by a method of claim 1.

$222 2? UNHTED STATEg PATENT @FFBCE QERTEFEQATJE @F RRETEN Patent No. 01 9 4956) Dated February 27, 1973 X Q fl MARINUS GERARDUS ANTOINE TAK ETAL It is eertifie'd that error appears in the above-identified patentand that said Letters Patent are hereby corrected as shown below:

In the title page, last line of the Abstract, "is" should read in Column6, line 52, EW ZN should read Eu Zr Signed and sealed this 16th day ofJuly 1973,,

(SEAL) Attest:

EDWARD. M.PLETCHER,JR. Rene Tegtmeyer Attesting Officer ActingCommissioner of Patents

2. A method of claim 1, wherein the mixture comprises between 1.75 and3.0 mol of ZrO2 per mol of disilicate.
 3. A method of claim 2, whereinthe mixture comprises between 1.5 and 2.5 mol of SiO2 per mol ofdisilicate.
 4. A method of claim 3 wherein the mixture is heated for 2to 8 hours at a temperature of between 1,300* and 1,400*C.
 5. A methodof claim 3, in which the barium strontium disilicate to be used ismanufactured by heating a starting mixture of barium oxide, strontiumoxide and silicon oxide, or of compounds which produce these oxides uponheating, wherein the starting mixture comprises at least 3 mol of SiO2per mol of barium oxide and strontium oxide so that the barium strontiumdisilicate formed is already mixed with quantity of SiO2 required forobtaining the desired luminescent silicate.
 6. A method of claim 3wherein the barium strontium disilicate to be used comprises the desiredquantity of europium.
 7. A luminescent silicate activated by bivalenteuropium and defined by the formula Ba1 x pSrxEupZrSi3O9 in which O < or= x < or = 0.2 and 0.002 < or = p < or = 0.10, and manufactured by amethod of claim 1.